Unconventional reservoirs include reservoirs such as tight-gas sands, gas and oil shales, coalbed methane, heavy oil and tar sands, and gas-hydrate deposits. These reservoirs have little to no porosity, thus the hydrocarbons may be trapped within fractures and pore spaces of the formation. Additionally, the hydrocarbons may be adsorbed onto organic material of a e.g. shale formation. Therefore, such reservoirs require special recovery operations outside the conventional operating practices in order to mobilize and produce the oil.
The rapid development of extracting hydrocarbons from these unconventional reservoirs can be tied to the combination of horizontal drilling and induced fracturing (call “hydraulic fracturing” or simply “fracking”) of the formations. Horizontal drilling has allowed for drilling along and within hydrocarbon reservoirs of a formation to better capture the hydrocarbons trapped within the reservoirs. Additionally, increasing the number of fractures in the formation and/or increasing the size of existing fractures through fracking increases hydrocarbon recovery.
In a typical hydraulic fracturing treatment, fracturing treatment fluid is pumped downhole into the formation at a pressure sufficiently high enough to cause new fractures or to enlarge existing fractures in the reservoir. Next, frack fluid plus a proppant, such as sand, is pumped downhole. The proppant material remains in the fracture after the treatment is completed, where it serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the well bore through the fracture. The spacing between fractures as well as the ability to stimulate fractures naturally present in the rock may be major factors in the success of horizontal completions in unconventional hydrocarbon reservoirs.
While there are a great many fracking techniques, one useful technique is “plug-and-perf” fracking. Plug-and-perf completions are extremely flexible multistage well completion techniques for cased hole wells. Each stage can be perforated and treated optimally because the fracture plan options can be modified in each stage. The engineer can apply knowledge from each previous stage to optimize treatment of the current fracture stage.
The process consists of pumping a plug-and-perforating gun to a given depth. The plug is set, the zone perforated, and the tools removed from the well. A ball is pumped downhole to isolate the zones below the plug and the fracture stimulation treatment is then pumped in, although washing, etching, and other treatments may occur first depending on downhole conditions. The ball-activated plug diverts fracture fluids through the perforations into the formation. After the fracture stage is completed, the next plug and set of perforations are initiated, and the process is repeated moving further up the well.
The key to reducing the cost of unconventional production using fracking is to optimize the fracturing parameters, namely well spacing, cluster spacing, job size, hydraulic fracture growth and geometry, and the like. Thus, the ability to monitor the geometry, spacing, orientation and length of the induced fractures is important to obtain optimal fracking. However, conventional monitoring methods, such as microseismic imaging, borehole gauges, or tracers provide very limited information about the fracture geometry and, hence, the effectiveness of the fracturing process and/or results.
Although hydraulic fracturing is quite successful, even incremental improvements in technology can mean the difference between cost effective production and reserves that are uneconomical to produce. Thus, what is needed in the art are improved methods of evaluating the hydraulic fracturing for every well being hydraulically simulated. Ideally, the improved methods would allow for monitoring the stimulation, as well as the ability to diagnose completion issues during the operation and monitor the production performance along complete well bore length